In this respect, refer to the document U.S. Pat. No. 5,355,294, which describes a DAB type conversion system, comprising an input rectifier connected to a power bus in which an alternating current flows, a DC/DC converter input delivering a first voltage level and a converter output providing a second voltage level and connected to the input converter by a transformer.
Such a system is intended to deliver in a cradle form two alternative voltages that are out of step with the other.
According to an application, this type of conversion system can be used to control the direction of the power transfer between the converter and a load based on the phase shift between the two voltages.
However, this type of conversion system requires using two converters, namely the input converter and the output converter. However, converters have a relatively large footprint which causes constraints on the use of this type of converter. In addition, the presence of two converters makes the power conversion system relatively expensive.
Also refer to document EP-A-2 006 993 that describes a bidirectional converter with H-bridge including several redundant bridges and in which each bridge can operate at different supply bus voltages to maximize the power output. During the normal operation of the converter, a voltage at seven different voltage levels is provided at the converter output in order to simulate a sinusoidal output voltage.
When a bridge is broken, the other bridges work in a three phase manner, providing output at five voltage levels.
On the other hand, document U.S. Pat. No. 6,005,788 describes a multilevel power converter for high voltage and high power applications including several inverters connected in series and with dedicated DC supply buses to provide multilevel waveforms. This converter thus requires the provision of many power buses.
Document U.S. Pat. No. 3,581,212 describes a quick response power conversion circuit including several inverters, each having two opposed output levels connected in series or in parallel and selectively controlled to produce a composite output.
When using voltages in an out of phase in a cradle form, as in the DAB converters, voltage levels must be kept at a nominal voltage level, close to the transformation ratio. If not, fluctuations appear in the current supplied to the load. Such fluctuations can be very large if the voltage difference becomes large.
FIGS. 1A, 1B, 1C and 1D illustrate the evolution of the current flowing between the load and the converter (curve I), the converter output voltage (curve II) and the load voltage (curve III), in case of low voltage difference (FIGS. 1A and 1B) and in case of high voltage difference (FIGS. 1C and 1D) both during a power transfer from the converter to the load (FIGS. 1A and 1C) and during a power transfer from the load to the converter (FIGS. 1B and 1D).
As seen on FIGS. 1a and 1b, a current fluctuation (curve I) appears even in case of low voltage difference between the voltage delivered by the converter (curve II) and the load voltage (curve III). These voltage fluctuations can be large when the difference in voltage increases (FIGS. 1C and 1D).
For applications embedded on board motor vehicles with at least partially electric traction, in which a two-way power transfer is implemented between the vehicle traction batteries and an inertia wheel, the power fluctuations are eliminated by using a bidirectional chopper placed between conversion stages. However, this chopper also constitutes an extra conversion stage that increases the volume and cost of the power conversion system. In addition, the chopper switching elements should be sized according to the maximum value of the current.